Bile salts are involved in the in vivo solubilization of fatty acids, phospholipids and cholesterol. They are known to dissolve gallstones, are involved in the transport of a variety of drugs across the intestinal wall, and deoxy bile salts are implicated in cancer. Thus it seems appropriate to study interactions and catalyses in bile salt systems. The self-association of various bile salts (e.g., sodium cholate, deoxycholate and taurocholate) and esters of bile acids will be investigated in aqueous and non-aqueous media using sedimentation equilibrium and vapor pressure osmometric techniques. From these studies we can establish the type of self-association present, the effect of changes in chemical structure on the self-association, and evaluate the equilibrium constants and nonideal terms. Concurrently, the structure of the aggregated bile salts, the interaction sites of solubilized drugs and other additives, as well as the microscopic and macroscopic environment of these sites will be investigated using H1 and C13 nmr spectroscopy, ultraviolet-visible spectrophotometry and other physical techniques. Due to the importance of bile acids and salts in digestive processes and transport across the intestinal wall, the catalysis (rate acceleration or inhibition) of the reactions of a variety of substrates and drugs will be investigated kinetically. Since the bile salts solubilize cholesterol and other lipids, dissolve gallstones, and are thought to be involved in drug absorption and transport, these studies are the first step in a study of the interactions of bile salts with these various substances.